In a recent study posted to the bioRxiv* pre-print server, researchers investigated how monoclonal antibodies (MAbs), S309 and AZD7442, reduced lung infection in mice infected with the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) variant of concern (VOC) Omicron’s BA.1, BA.1.1, and BA.2 sublineages.
S309, the parent of Sotrovimab, is a cross-reactive SARS-CoV MAb, whereas the AZD7442, clinically known as Evusheld, is a combination of AZD8895 and AZD1061 MAbs. While the S309 binds to a conserved epitope on the receptor-binding domain (RBD) of SARS-CoV-2 spike (S) glycoprotein, the Evusheld binds to non-overlapping receptor-binding motif (RBM) epitopes. Although previous studies reported that several MAbs lose their neutralizing potency against Omicron sublineages in vitro, their effects in vivo are unknown.
About the study
In the current study, researchers assessed the in vitro neutralizing activity of S309, AZD8895, AZD1061, and AZD7442 MAbs against BA.1, BA.1.1, and BA.2 sublineages in Vero-transmembrane serine protease 2 (TMPRSS2) cells. To this end, they used mAbs with fragment crystallizable (Fc) modifications, S309-LS, AZD8895-YTE/TM, AZD1061-YTE/TM, and AZD7442-YTE/TM, that extend antibody half-life in humans and reduce Fc effector functions.
For the in vivo studies, they used S309-LS and AZD7442 MAbs, with only the TM mutations (not the YTE mutations). The research team intraperitoneally injected a 200 μg mAb dose in human keratin 18 (K18)-human angiotensin-converting enzyme 2 (hACE2) transgenic mice infected with BA.1, BA.1.1, or BA.2 sublineages, and harvested sera samples six days post-infection (dpi) for BA.2, and seven dpi for BA.1.1 and BA.1. Notably, all these sublineages replicate rapidly in the lungs of K18-hACE2 mice.
Findings
Compared to the ancestral D614G SARS-CoV-2 strain, incubation of S309-LS, AZD7442-YTE/TM, and AZD8895-YTE/TM, AZD1061-YTE/TM antibodies with BA.1 sublineage reduced their in vitro neutralization potency by 2.5-fold, 25-fold, 118-fold, and 206-fold, respectively.
Although S309-LS and AZD8895-YTE/TM were slightly less effective against BA.1.1 than BA.1 in vitro, the authors observed a 1700-fold reduction in the neutralizing potency of AZD1061-YTE/TM. Yet, the AZD7442-YTE/TM combination showed some inhibitory activity against the BA.1.1 variant.
Overall, S309 retained in vitro neutralizing potency against BA.1 and BA.1.1 sublineages but had reduced in vitro neutralizing activity against BA.2, whereas AZD7442 combination showed reduced yet some neutralizing activity against all three Omicron lineages.
In BA.1 and BA.1.1-infected mice, S309-LS mAb reduced viral burden in the lung and nasal turbinates at seven dpi compared to control group mice treated with isotype mAb. The control group K18-hACE2 mice had increased expression of several cytokines and chemokines, including granulocyte-macrophage colony-stimulating factor (GM-CSF), interferon-gamma (IFN-γ), interleukin-1-beta (IL-1β), and interleukin 6 (IL-6). However, BA.1 or BA.2-infected mice (but not BA.1.1) treated with AZD7442-TM mAb had reduced pro-inflammatory cytokine and chemokine expression.
Despite their low in vitro neutralization potency against all Omicron sublineages, S309-LS and AZD7442-TM efficiently reduced viral burden and cytokine levels in the lungs of the infected mice. S309-LS treatment resulted in a 742-fold reduction in viral ribonucleic acid (RNA) in the lungs of BA.2-infected mice; however, it could not reduce the viral burden in the nasal turbinates of these mice.
AZD7442-TM treatment reduced viral burden in the lungs by 91-fold, four-fold, and >100,000-fold in BA.1, BA.1.1, and BA.2-infected mice, respectively. Similar to S309-LS, its protective effect remained limited on the viral burden in the nasal washes of infected mice, similar to how their parent MAbs COV2-2196 and COV2-2130 showed less protection in nasal washes against several SARS-CoV-2 VOCs.
Several studies in non-human primates have shown that the concentrations of MAbs in nasal washes are approximately 0.1% of those found in the serum, thus rationalizing the diminished effect of these MAbs in these tissues.
For the AZD7442-TM, with Fc modifications, antibody-mediated reductions in lung viral titers directly correlated with its neutralization activity against the Omicron sublineages. For S309-LS, changes in the neutralization potency did not linearly correlate to changes in viral titers. Nevertheless, it conferred adequate protection in the lungs of BA.2-infected mice despite a substantial loss in the neutralizing activity in vitro.
Conclusions
The study built upon recent in vitro findings of BA.1 sublineages and showed the level of protection conferred against all three circulating Omicron sublineages by treatment with two MAbs, S309 and AZD7442-TM.
The findings showed that while S309-LS and AZD7442-TM retained inhibitory activity against these Omicron sublineages, their altered in vitro neutralization potency did not directly impact their in vivo potency and dosing in the clinical setting.
The study also highlighted that for these MAbs, Fc effector functions compensated for the loss in the neutralization potency against SARS-CoV-2 variants and worked as a protective mechanism in vivo.
Thus, despite losses in neutralizing potency in cell culture, S309-LS or AZD7442-TM prevented inflammation in the lungs of mice infected with Omicron variants.
Overall, the study experiments remarkably demonstrated how Fc effector functions could contribute to the resilience of some mAbs against Omicron and other VOCs.
*Important notice
bioRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.
